Burner control system

ABSTRACT

A burner control system that locks out for flame failure but recycles to an ignition sequence after a power failure. Recycling can occur even if a flame is momentarily present in the combustion chamber when power is restored, but the system locks out if the flame is not extinguished within a predetermined period of time. Included in the control system is a control circuit with a lockout switch actuating circuit including a latchable switch means that closes when there is no flame and an energizing signal is applied, remains closed for the duration of the energizing signal, and opens when the signal has terminated. Closing of the latchable switch means is triggered by a control relay, and the control circuit is electrically isolated from greater voltages in the output of a flame sensor, thereby causing the latchable switch means to open at least as soon as the control relay is deenergized when an energizing signal is lost. A circuit to lockout for flame failure or a dangerously long flame persistence after power failure is also provided.

United States Patent [1 1 Cade I 51 Aug. 20, 1974 I 1 BURNER CONTROL SYSTEM Phillip J. Cade, Winchester, Mass.

[73] Assignee: Electronics Corporation of America, Cambridge, Mass.

22 Filed: May 4,1973

21 App1.No.:357,457

[75] Inventor:

[52] U.S. Cl. 431/78 [51] Int. Cl. F23n 5/00 [58] Field of Search 431/78. 46, 79, 80

[56] References Cited UNITED STATES PATENTS 3.472320 10/1969 Willson 431/78 X 3.705.783 12/1972 Warren... 431/78 Primary E.\'uminerEdward G. Favors [57] ABSTRACT A burner control system that locks out for flame failure but recycles to an ignition sequence after a power failure. Recycling can occur even if a flame is momentarily present in the combustion chamber when power is restored, but the system locks out if the flame is not extinguished within a predetermined period of time.

Included in the control system is a control circuit with a lockout switch actuating circuit including a latchable switch means that closes when there is no flame and an energizing signal is applied, remains closed for the duration of the energizing signal, and opens when the signal has terminated. Closing of the latchable switch means is triggered by a control relay, and the control circuit is electrically isolated from greater voltages in the output of a flame sensor, thereby causing the latchable switch means to open at least as soon as the control relay is de-energized when an energizing signal is lost. A circuit to lockout for flame failure or a dangerously long flame persistence after power failure is also provided.

14 Claims, 1 Drawing Figure MA IZUK PAIENTEBmczoxsn I kQNM kbm BURNER CONTROL SYSTEM BACKGROUND OF THE INVENTION This invention relates to electrical control circuitry and more particularly to control circuitry adapted for use in burner control systems.

In designing burner control systems, various safeguards must be taken to ensure that the burner is not operated in a dangerous manner. For example, should a sudden loss of flame occur it is essential that the flow of fuel to the combustion chamber be shut off to prevent an explosive accumulation of fuel. Similarly, it is dangerous to initiate an ignition sequence if a flame is already present in the chamber. While a burner control system should be designed to lockout under such conditions, there is usually no need to go to lockout for momentary power interruptions such as may be caused by circuit breaker action on external power lines feeding the burner. Lockout is not called for even if a flame remains in the combustion chamber for a short time after power restoration, except when the flame fails to be extinguished and continues to burn for a period longer than the operating time of the lockout apparatus. Such a flame may be caused by residual fuel remaining in the combustion chamber or fuel inlet after the inception of the power failure.

A problem encountered in the prior art was the tendency of a control system to go to lockout after a power failure if flame was present when power was restored, even though the flame might thereafter consume the remaining residual fuel and be extinguished before renewed burner operation could create a dangerous situation. Instead of automatically restarting, the burner would be idled until the cause of the lockout could be investigated and the lockout mechanism reset. In order to provide a recycle capability after power failure, rather complicated circuitry has been resorted to, and the ability of the control system to go to lockout should the flame not be rapidly extinguished has been compromised.

SUMMARY OF THE lNVENTION In accordance with the above, it is an object of this invention to provide an improved burner control system that automatically recycles burner operation to an ignition sequence after momentary loss of power. Another object is to provide such a control system that locks out burner operation following power interruption should a flame persist in the combustion chamber for a dangerously long period of time after power has been restored. A further object is to provide such a burner control system that locks out burner operation and does not recycle to an ignition sequence for loss of flame.

In accordance with the invention, there is provided burner control apparatus for use with a fuel burner installation that has a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, and means for applying an energizing signal to the burner control apparatus in response to a request for burner operation from the operating control. The apparatus includes a lockout switch to shut down burner operation and control circuitry connected to a.

pair of input terminals. A circuit for actuating the lockout switch is included in the control circuit and has latchable switch means connected in an actuating path, with means provided to close the latchable switch means in response to the application of an energizing signal at the input terminals. A circuit is provided to hold the latchable switch means closed for the duration of the energizing signal, the latchable switch means being adapted to open in response to the loss of an energizing signal and thereby prevent the completion of a lockout switch actuating path following a momentary power failure. The actuating path also includes an additional switch in series with the latchable switch means, the additional switch being respectively opened and closed in response to flame and no-flame indications from the flame sensor.

In another aspect of the invention, electrical storage means are connected to smooth and maintain the supply voltage for the flame sensor circuitry. Isolating means enable the signal at the input terminals to drop rapidly in response to an interruption in the energizing signal, while the voltage level is maintained at the flame sensor circuit. Rapid opening of the latchable switch means in response to a power failure is thereby achieved, while the latchable switch means remains closed during flame failure as long as an energizing signal is present. 7

In another aspect of the invention a control relay is provided that is energized in response to the application of an energizing signal if no flame is sensed. Rapid response means operates at least as-fast as the drop-out time of the control relay to remove a holding signal from the latchable switch means when an energizing signal is lost. The latchable switch means is thereby caused to open and prevent completion of the lockout actuating path at least as soon as the control relay is deenergized.

Another aspect of the invention deals with triggering the latchable switch means to close, and involves the use of trigger means that are energized a predetermined period of time after the occurrence of the combination of a request for burner operation from the op,- erating control together with a no-flame indication from the flame sensor. In a particular embodiment the trigger means includes the control relay in combination with a'set of control relay contacts that are connected to trigger the latchable switch means to close when the control relay is energized.

Other features of the invention include a flame relay that is also provided with energizing current from the input terminals and is connected in series to hold in, but not initially energize, the control relay when flame is sensed, and'a second lockout switch actuating path comprising the series combination of a switch that is respectively closed and opened inresponse to flame and no-flame indications from the flame sensor, and

- another switch that is respectively closed and opened in response to the absence or presence of an energizing signal. In a particular embodiment the first-mentioned switch comprises a set of normally open flame relay contacts and the other switch comprises a set of 'normally closed control relay contacts. With this arrangement both relays drop out for a momentary flame loss. Only the flame relay is re-energized when the flame returns, closing the normally open flame contacts 'to complete the second lockout switch actuatingpath and shut down burner operation. The second actuating path also spark ignition circuitry 28 and the pilot fuel control 29. The resulting fuel flow is ignited and the flame sensor 64 completes a circuit through secondary windings l8 and of transformer 16 when it senses the pilot flame. The signal from flame sensor 64 is coupled to the amplifier circuit via inductor 74 to switch on transistors 92 and 94. The flame relay 100 is then energized from rectifier terminal 54 through diode 96 and conducting transistor 94. Contacts 100-1 close, permitting fuel to flow to the main burner unit; contacts 100-2 open, terminating ignition; contacts 100-3 close, discharging capacitor 114 in a resetting operation; contacts 100-4 close; and contacts 10.0-5 open, terminating the heating of lockout heater 102. SCR 104 is latched closed through resistor 120 when contacts 100-5 open, and control relay 98 is held in by the current flow through flame relay 100.

Should flame not be established within a predetermined interval as determined by the characteristics of lockout heater 102, that heater will open contact 102-2, de-energizing the main burner control components 27, 28, 29, and 30 and the control transformer 36, shutting down the system in safe condition; and close contacts 102-l to energize alarm 26. lf flame is established, operating control 24 operates in a similar manner after the demand for heat has been satisified to de-energize the main combustion control components 27, 28, 29, and 30 and to de-energize the control transformer 36.

Should an established flame be interrupted while transformer 36 is still energized in response to a request for burner operation, flame sensor 64 will stop conducting, causing amplifier circuit transistors 92 and 94 to cease conducting and thereby de-energize flame relay 100. Flame relay contacts 100-5 close to complete the same heating circuit for lockout heater 102 as during startup when control relay 98 has been energized but before flame relay 100 has been energized, i.e.. through flame relay contacts 100-5, SCR 104, and the 12 ohm tap of the control relay 98 coil. The SCR is held in through resistor 120 until flame relay 100 drops out and contacts 100-5 close. Control relay 98, held in through the flame relay circuit during safe burner operation. is timed to drop out upon loss of flame slightly before flame relay 100 drops out. The heating circuit established when the flame relay 100 subsequently drops out shunts the RC timing network to prevent capacitor 114 from recharging sufficiently to fire unijunction 118. After the predetermined time period lockout heater 102 will operate to shut down burner operation and energize alarm 26 as described above.

If the flame should reappear before the lockout heater has operated, the flame relay will be reenergized and open contacts 100-5 to break the heating circuit. At the same time, however, contacts 100-4 will close to complete a heating circuit through closed control relay contacts 98-2 (control relay 98 is still deenergized because energization of flame relay 100, while sufficient to hold in the control relay, is not adequate to energize the control relay). The heating cycle will then continue as before.

In the event of a momentary power failure that causes an interruption in an established request for burner operation signal, it may be necessary to shut the system down if a flame persists. Because of residual fuel remaining in the combustion area or in the fuel lines after the relays have been de-energized and have shut off the fuel supply, it is possible that a flame will continue for a short time afterwards. If the flame is still present when power is restored, a potentially hazardous situation is created. The danger is alleviated, however, if the flame goes out shortly thereafter. System lockout becomes necessary only if the flame persists for an extended period after power is restored. In the present invention the burner is recycled to a normal ignition sequence following momentary power failures, with lockout only for a persistent flame, even if a flame is briefly present when power is restored. When the energizing signal at the input terminals 54, 56 is lost due.to a power failure or the like, current through the coils of control relay 98 and flame relay l00will fall off and the relays will drop out, delayed somewhat by the electrical storage effect of smoothing capacitor 126 and mechanical delays inherent in the relays. By the time the relays can drop out the signal applied to the SCR 104, shunted by the 10 microfared capacitor 112 and isolated by diode 96 from the higher voltage maintained at the flame sensor output circuit, will have fallen off sufficiently to cause the SCR to cease conducting. When power returns, any flame still existing will be detected by flame sensor 64 and flame relay 100 will be energized, closing contacts 1004 to form an actuating path for lockout heater 102 through contacts 100-4 and 98-2. The current through the flame relay 98 alone is insufficient to energize control relay 100, and flame relay contacts 100-3 will close to prevent charging of trigger capacitor 114. The heater will continue to heat for as long as the flame lasts, and will lockout the burner system after the normal heating period elapses. Should the flame die out before this time, however, flame relay 100 will drop out, opening contacts 100-4 to break the heating path. Contacts 100-5 will close, but closed contacts 98-2 will have grounded the gate of SCR 104 to hold the SCR in a nonconducting state since the control relay first dropped out, thereby preventing the completion of an alternate heating path. Contacts 100-3 also open, permitting capacitor 114 to charge until it has reached a sufficient value to fire the unijunction transistor 118 and energize the control relay 98. Operation of the burner system will then continue as described above for a normal ignition sequence.

While a particular embodiment of the invention has been shown and described, various modifications thereof will be apparent to those skilled in the art. Therefore it is not intended that the invention be limited to the disclosed embodiment or to details thereof and departures may be made therefrom within the spirit and scope of the invention as defined in the claims.

What is claimed is:

1. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, and means for applying an energizing signal to the burner control apparatus in response to a request for burner operation from said operating control, comprising:

a lockout switch to'shut down burner operation, and

control circuitry connected to a pair of input terminals, said control circuitry including a control relay energized in response to the application of an energizing signal at said input terminals while said flame sensor indicates lack of flame,

a lockout switch actuating circuit including latchable switch means connected in an actuating path, means operable in response to the combination of an energizing signal applied at said input terminals and the absence of a flame indication from said flame sensor for closing said latchable switch means, a circuit for applying a latching signal to hold said latchable switch means closed for the duration of an applied energizing signal, said latchable switch means adapted to open in response to the loss of an energizing signal at said energizing terminals,

a flame relay connected in series with said control relay, and normally closed contacts controlled by said flame relay connected in said actuating path in series with said latchable switch means, said flame relay connected to said energizing terminals for energization when said flame sensor indicates flame, the control relay-flame relay series circuit being characterized by an electrical impedance sufficient to maintain said control relay in an energized state when an energizing signal is present at said terminals but insufflcient to enable energization of the control relay from an initial deenergized state, and

rapid response means for removing a latching signal from said latchable switch means when an energizing signal is lost from said energizing terminals, and thereby causing said latchable switch means to open, said rapid response means operable at least as fast as the drop-out time of said control relay to open said latchable switch means at least as fast as said control relay is de-energized.

2. Burner control apparatus as claimed in claim 1, further including a second actuating path in said lockout switch actuating circuit, the second actuating path including normally closed control relay contacts in series with normally open flame relay contacts.

3. Burner control apparatus as claimed in claim 1, wherein said means for closing the switch means comprises control relay contacts connected to trigger said latchable switch means to close when said control relay is energized.

4. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control. a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, an output circuit for said flame sensor connected to a pair of input terminals for the burner control apparatus, electrical storage means connected to smooth and maintain the signal at said output circuit during fluctuations in the signal at said input terminals, and means for applying an energizing signal at said input terminals in response to a request for burner operation from said operating control, comprising: a lockout switch to shut down burner operation, control circuitry connected to said input terminals,

said control circuitry including a lockout switch actuating circuit, and latchable switch means in said actuating circuit connected in an actuating path, said latchable switch means when closed adapted to remain closed during the continuation of an energizing signal at said input terminals and to open upon the termination of such an energizing signal, and

means to isolate said control circuitry from voltage levels across said flame sensor output circuit that are greater than the energizing signal applied at said input terminals, thereby enabling the signal at said input terminals to drop rapidly and said latchable switch means to rapidly open in response to a loss of energizing signal.

5. Burner control apparatus as claimed in claim 4, further including a second electrical storage means of lesser capacity than said flame sensor output circuit storage means, connected to smooth the signal at said latchable switch means, and a control relay energized in response to the application of an energizing signal at said input terminals while said flame sensor indicates lack of flame, said second electrical storage means adapted to rapidly reduce the energizing signal at said latchable switch means when an energizing signal is lost at said input terminals, thereby enabling said latchable switch means to open upon loss of an energizing signal at said input terminals at least as soon as said control relay deenergizes.

6. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, and means for applying an energizing signal to the burner control apparatus in response to a request for burner operation from said operating control, comprising:

a lockout switch to shut down burner operation, and

control circuitry connected to a pair of input terminals, said control circuitry including a lockout switch actuating circuit,

latchable switch means in said actuating circuit connected in an actuating path, said latchable switch means when closed adapted to remain closed during the continuation of an energizing signal at said energizing terminals,

trigger means for closing said latchable switch means,

and

a circuit for energizing said trigger means a predetermined period of time after the occurrence of the combination of a request for burner operation from said operating control together with a no-flame indication from said flame sensor,

7. Burner control apparatus as claimed in claim 6, wherein said trigger means comprises a control relay in combination with a set of control relay contacts, said contacts connected to said latchable switch means to trigger closing thereof when said control relay is energized.

8. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presenceof flame in the combustion chamber, and means for applying an energizing signal to the burner control apparatus in response to a request for burner operation from said operating control, comprising:

a lockout switch to shut down burner operation, control circuitry connected to a pair of input terminals, including an actuating circuit for said lockout switch, said actuating circuit including first and second switches connected in series to form a first actuating path, and third and fourth switches connected in series to form a second actuating path,

said first switch respectively opened and closed in response to flame and noflame indications from the flame sensor,

said second switch closed in response to an energizing signal applied at said input terminals, held closed for the duration of an energizing signal, and opened in response to loss of an energizing signal at the input terminals,

said third switch respectively closed and opened in response to flame and no-flame indications from said flame sensor, and

said fourth switch respectively closed and opened in response to the absence or presence of an energizing signal at said input terminals.

9. Burner control apparatus as claimed in claim 8, including a trigger circuit to apply a closing signal from said input terminals to said second switch, wherein said fourth switch is connected to said trigger circuit to bypass said second switch and prevent the application of a closing signal thereto during the time that said fourth switch is closed.

10. Burner control apparatus as claimed in claim 8 and further including a first relay for operating said fuel control, circuitry for energizing said first relay in response to an energizing signal applied at said input terminals, a second relay responsive to a signal from said flame sensor indicating the presence of flame in the combustion chamber, and circuitry for opening said second switch prior to release of said first and second relays in response to loss of an energizing signal at said input terminals.

11. Burner control apparatus as claimed in claim 10 wherein said release allowing circuitry includes an asymmetrically conductive device connected in series between said input terminals and said first and second relays and a capacitor for delaying release of said first and second relays upon loss of an energizing signal at said input terminals.

12. Burner control apparatus as claimed in claim 11 and further including time delay circuit responsive to an energizing signal applied at said input terminals for delaying energization of said first relay; and a fifth switch respectively closed and opened in response to flame and no flame indications from said flame sensor, said fifth switch being connected to said time delay circuitry to prevent completion of a time delay interval to energize said first relay during the presence of a flame indication from said flame sensor; and wherein said second switch is a controlled rectifier and said fourth switch is opened in response to energization of said first relay.

l3. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, and means for applying an energizing signal to the burner control apparatus in response to a request for burner operation from said operating control, comprising:

a lockout switch to shut down burner operation,

control circuitry connected to a pair of input terminals,

a lockout switch actuating circuit in said control circuitry, said actuating circuit including a silicon controlled rectifier, and a switch respectively opened and closed in response to flame and noflame indications from said flame sensor, the anode-cathode circuit of said silicon controlled rectifier connected in series with said switch to form a lockout switch actuating path,

a trigger circuit connected to the gate of the silicon controlled rectifier, said trigger circuit including a trigger switch controlling the application of a gating signal from said input terminals to said gate, and

means for operating said trigger switch in response to a request for burner operation from said operating control to apply a gating signal to said silicon controlled rectifier.

14. Burner control apparatus as claimed in claim 13, further including a switch respectively closed and opened in response to flame and no-flame indications from said flame sensor, said switch connected in series with said trigger switch to form a second actuating path for the lockout switch. 

1. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, and means for applying an energizing signal to the burner control apparatus in response to a request for burner operation from said operating control, comprising: a lockout switch to shut down burner operation, and control circuitry connected to a pair of input terminals, said control circuitry including a control relay energized in response to the application of an energizing signal at said input terminals while said flame sensor indicates lack of flame, a lockout switch actuating circuit including latchable switch means connected in an actuating path, means operable in response to the combination of an energizing signal applied at said input terminals and the absence of a flame indication from said flame sensor for closing said latchable switch means, a circuit for applying a latching signal to hold said latchable switch means closed for the duration of an applied energizing signal, said latchable switch means adapted to open in response to the loss of an energizing signal at said energizing terminals, a flame relay connected in series with said control relay, and normally closed contacts controlled by said flame relay connected in said actuating path in series with said latchable switch means, said flame relay connected to said energizing terminals for energization when said flame sensor indicates flame, the control relay-flame relay series circuit being characterized by an electrical impedance sufficient to maintain said control relay in an energized state when an energizing signal is present at said terminals but insufficient to enable energization of the control relay from an initial deenergized state, and rapid response means for removing a latching signal from said latchable switch means when an energizing signal is lost from said energizing terminals, and thereby causing said latchable switch means to open, said rapid response means operable at least as fast as the drop-out time of said control relay to open said latchable switch means at least as fast as said control relay is de-energized.
 2. Burner control apparatus as claimed in claim 1, further including a second actuating path in said lockout switch actuating circuit, the second actuating path including normally closed controL relay contacts in series with normally open flame relay contacts.
 3. Burner control apparatus as claimed in claim 1, wherein said means for closing the switch means comprises control relay contacts connected to trigger said latchable switch means to close when said control relay is energized.
 4. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, an output circuit for said flame sensor connected to a pair of input terminals for the burner control apparatus, electrical storage means connected to smooth and maintain the signal at said output circuit during fluctuations in the signal at said input terminals, and means for applying an energizing signal at said input terminals in response to a request for burner operation from said operating control, comprising: a lockout switch to shut down burner operation, control circuitry connected to said input terminals, said control circuitry including a lockout switch actuating circuit, and latchable switch means in said actuating circuit connected in an actuating path, said latchable switch means when closed adapted to remain closed during the continuation of an energizing signal at said input terminals and to open upon the termination of such an energizing signal, and means to isolate said control circuitry from voltage levels across said flame sensor output circuit that are greater than the energizing signal applied at said input terminals, thereby enabling the signal at said input terminals to drop rapidly and said latchable switch means to rapidly open in response to a loss of energizing signal.
 5. Burner control apparatus as claimed in claim 4, further including a second electrical storage means of lesser capacity than said flame sensor output circuit storage means, connected to smooth the signal at said latchable switch means, and a control relay energized in response to the application of an energizing signal at said input terminals while said flame sensor indicates lack of flame, said second electrical storage means adapted to rapidly reduce the energizing signal at said latchable switch means when an energizing signal is lost at said input terminals, thereby enabling said latchable switch means to open upon loss of an energizing signal at said input terminals at least as soon as said control relay deenergizes.
 6. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, and means for applying an energizing signal to the burner control apparatus in response to a request for burner operation from said operating control, comprising: a lockout switch to shut down burner operation, and control circuitry connected to a pair of input terminals, said control circuitry including a lockout switch actuating circuit, latchable switch means in said actuating circuit connected in an actuating path, said latchable switch means when closed adapted to remain closed during the continuation of an energizing signal at said energizing terminals, trigger means for closing said latchable switch means, and a circuit for energizing said trigger means a predetermined period of time after the occurrence of the combination of a request for burner operation from said operating control together with a no-flame indication from said flame sensor.
 7. Burner control apparatus as claimed in claim 6, wherein said trigger means comprises a control relay in combination with a set of control relay contacts, said contacts connected to said latchable switch mEans to trigger closing thereof when said control relay is energized.
 8. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, and means for applying an energizing signal to the burner control apparatus in response to a request for burner operation from said operating control, comprising: a lockout switch to shut down burner operation, control circuitry connected to a pair of input terminals, including an actuating circuit for said lockout switch, said actuating circuit including first and second switches connected in series to form a first actuating path, and third and fourth switches connected in series to form a second actuating path, said first switch respectively opened and closed in response to flame and no-flame indications from the flame sensor, said second switch closed in response to an energizing signal applied at said input terminals, held closed for the duration of an energizing signal, and opened in response to loss of an energizing signal at the input terminals, said third switch respectively closed and opened in response to flame and no-flame indications from said flame sensor, and said fourth switch respectively closed and opened in response to the absence or presence of an energizing signal at said input terminals.
 9. Burner control apparatus as claimed in claim 8, including a trigger circuit to apply a closing signal from said input terminals to said second switch, wherein said fourth switch is connected to said trigger circuit to bypass said second switch and prevent the application of a closing signal thereto during the time that said fourth switch is closed.
 10. Burner control apparatus as claimed in claim 8 and further including a first relay for operating said fuel control, circuitry for energizing said first relay in response to an energizing signal applied at said input terminals, a second relay responsive to a signal from said flame sensor indicating the presence of flame in the combustion chamber, and circuitry for opening said second switch prior to release of said first and second relays in response to loss of an energizing signal at said input terminals.
 11. Burner control apparatus as claimed in claim 10 wherein said release allowing circuitry includes an asymmetrically conductive device connected in series between said input terminals and said first and second relays and a capacitor for delaying release of said first and second relays upon loss of an energizing signal at said input terminals.
 12. Burner control apparatus as claimed in claim 11 and further including time delay circuit responsive to an energizing signal applied at said input terminals for delaying energization of said first relay; and a fifth switch respectively closed and opened in response to flame and no flame indications from said flame sensor, said fifth switch being connected to said time delay circuitry to prevent completion of a time delay interval to energize said first relay during the presence of a flame indication from said flame sensor; and wherein said second switch is a controlled rectifier and said fourth switch is opened in response to energization of said first relay.
 13. Burner control apparatus for use with a fuel burner installation having a combustion chamber, an operating control, a fuel control for controlling the flow of fuel to the combustion chamber, an ignition control for igniting fuel in the combustion chamber, a flame sensor for sensing the presence of flame in the combustion chamber, and means for applying an energizing signal to the burner control apparatus in response to a request for burner operation from said operating control, comprising: a lockout switch to shut down burner operation, control circuitry cOnnected to a pair of input terminals, a lockout switch actuating circuit in said control circuitry, said actuating circuit including a silicon controlled rectifier, and a switch respectively opened and closed in response to flame and no-flame indications from said flame sensor, the anode-cathode circuit of said silicon controlled rectifier connected in series with said switch to form a lockout switch actuating path, a trigger circuit connected to the gate of the silicon controlled rectifier, said trigger circuit including a trigger switch controlling the application of a gating signal from said input terminals to said gate, and means for operating said trigger switch in response to a request for burner operation from said operating control to apply a gating signal to said silicon controlled rectifier.
 14. Burner control apparatus as claimed in claim 13, further including a switch respectively closed and opened in response to flame and no-flame indications from said flame sensor, said switch connected in series with said trigger switch to form a second actuating path for the lockout switch. 